1. Field of the Invention
The present invention relates to an aqueous coating system manufacturing method for a photothermographic material, and a photothermographic material prepared therewith. More specifically, the invention relates to an aqueous coating system manufacturing method for a photothermographic material of high image quality excellent in coating stability and with improved haze in images, as well as a photothermographic material prepared therewith.
2. Description of the Related Art
In recent years, decreasing the amount of processing liquid waste in the field of films for medical imaging has been desired from the viewpoints of protecting the environment and economy of space. Technology is therefore required for photosensitive thermal developing image recording materials which can be imagewise exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications. An image forming system using photosensitive thermal developing image recording materials does not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Photothermographic materials utilizing organic silver salts are described in many documents. Photothermographic materials generally have an image forming layer including a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images, dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. The Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system that has been made commercially available.
Methods of manufacturing such photothermographic material include a method of manufacture by a solvent coating, and a method of coating an aqueous coating solution using an aqueous dispersion of fine polymer particles or an aqueous solution of a water soluble polymer as a main binder followed by drying. Since the latter method does not require a process of solvent recovery or the like, a production facility therefor is simple, environmental burden is small, and the method is advantageous for mass production.
However, in the method of manufacturing the photothermographic material by an aqueous coating system, since the coating solution for the image forming layer contains many components required for image formation, there is a significant problem with regard to uniformly coating and drying the same. Particularly, in a case of coating a solution at a high speed and rapidly drying the same to prepare a photothermographic material in order to enhance productivity, there are various problems such as increase of haze due to partial lack of balance among the components in the coated layer and occurrence of unevenness in the coated surface state due to fluctuation of drying wind.
Further, in a case where the binder of the image forming layer is a polymer latex, since the coating solution has no setting property, these problems are significant. “Having setting property” in the present specification means that a coating solution can form a state in which it does not substantially deform physically even when an external pressure such as drying wind is applied when it is coated in a sol state and then the temperature is lowered below a gelation temperature, in a case where the coating solution has “sol-gel transformability”, that is, a case where the coating solution attains a sol state and maintains fluidity when the temperature thereof is equal to or higher than a certain temperature (usually, about 30° C.) but gels and loses fluidity at a temperature lower than the certain temperature.
Since the unevenness in the coated surface state results in image unevenness when images are formed and the increase in haze deteriorates the image quality, these are significant problems that particularly affect diagnosis ability for image forming materials for medical use, and improvement has always been demanded.
As means for improvement, the specification of Japanese Patent Application Laid-Open (JP-A) No. 10-186568 discloses use of a polymer having setting property as a binder for a surface protective layer disposed above the image forming layer and coating the same by a simultaneous double-layer coating method together with the image forming layer.
In addition, while the specification of JP-A No. 10-186571 discloses a method of improving the coated surface state by providing a non-photosensitive layer between the image forming layer and the surface protective layer, and the specification of JP-A No. 11-133543 discloses a method of using a hardener for the surface protective layer, none of these methods have been satisfactory.